1. Field of Invention
The invention relates to the field of semiconductors. More particularly, the invention is directed to nitride semiconductor films for use in blue light emitting devices.
2. Description of Related Art
Semiconductor light emitters have many applications: light-emitting diodes (LEDs) are used, for instance, for displays and lighting applications; laser diodes (LDs), which emit coherent light, are used in telecommunications, data storage, and for printing.
These devices emit light when a current passes through a pn-junction. As shown in FIG. 1, the diode 100 includes a sandwich of a p-type 110 semiconductor material and an n-type 120 semiconductor material. These materials are characterized by a bandgap E.sub.g.sup.1 130. The bandgap 130 is the energy difference between the valence band 140 and the conduction band 150. The bandgap determines the energy of a photon produced when an electron in the conduction band recombines with a hole in the valence band. When current passes through the diode 100, the electrons 160 in the conduction band 150 flow across the junction from the n-type material 120, while the holes 170 from the valence band 140 flow from the p-type material 120. As a result, a significant number of the electrons 160 and holes 170 recombine in the p-n junction. In order to enhance the recombination efficiency, an active layer 180 with lower bandgap E.sub.g.sup.2 190 is often included at the center of the device. The lower bandgap in this so-called active layer 180 leads to efficient trapping of electrons and holes in the same spatial region and to more efficient recombination of electrons and holes. This type of device structure is called a double heterostructure. If the thickness of the active layer 180 is small enough for quantum mechanical confinement effects to be important, the active layer is called a quantum well.
The wavelength, and thus the color, of light emitted by an LED or laser diode depends on the bandgap of the active layer E.sub.g.sup.2. LEDs or laser diodes that emit light in the infrared or in the red-to-yellow spectrum have been available for many years. There has been great difficulty in developing semiconductor light emitters at shorter wavelengths. Extending LED light sources into the short-wavelength region of the spectrum, the region extending from green to violet, is desirable because LEDs can then be used to produce light in all three primary colors. Shorter-wavelength laser diodes will also permit the projection of coherent radiation to focus laser light into smaller spots. That is, in the optical diffraction limit, the size of a focused spot is proportional to the wavelength of the light. This allows high-density optical information to be stored and read out.